Applicants' invention relates to communications systems including transmitters and receivers, and more particularly to a communications system that optimizes performance by dynamically adjusting transmitter power and receiver attenuation.
In a typical telecommunications system, such as in a cellular telephone system, a receiver is designed to function with signals in a given signal strength range. This signal strength range is referred to as the dynamic range of the receiver. The lowest acceptable signal strength that can be received is called the receiver sensitivity. The highest acceptable signal strength that can be received is called the blocking limit.
In a cellular telephone system, the distance between a base station (BS) and a mobile station (MS) varies as the mobile station moves around within a cell. Consequently, in large cells, the variations in received signal strength can be very large. Sometimes, these variations are so large that they cannot be compensated by varying the mobile station's transmit power. In such cells, it would be useful for the receiver dynamic range, that is, the signal level range defined by the highest and lowest acceptable received signal levels, to be changed/transposed in relation to the distance between the base station and the mobile station.
The receiver dynamic range may be transposed by placing an attenuator between the antenna and the receiver. As attenuation increases, both the lowest and highest acceptable received signal strengths increase. Similarly, the receiver dynamic range is transposed downward as attenuation decreases. This influence of attenuation on dynamic range is illustrated in FIG. 1. When attenuation is set at 0 decibels (dB), the receiver dynamic range 101 is defined by the sensitivity 102 and the blocking limit 103 of the receiver (not shown).
If attenuation is increased to 3 dB, the receiver dynamic range shifts to a new dynamic range 104, defined by sensitivity 105 and blocking limit 106, both of which are substantially 3 dB higher than the respective sensitivity 102 and blocking limit 103 that existed when attenuation is only 0 dB. Similarly, if attenuation is increased to 6 dB, the sensitivity 108 and blocking limit 109 are again increased, yielding a new receiver dynamic range 107.
In small cells, the distance between a mobile station and a base station can be very small. This can lead to disturbances in the receive band if there are large cells in the same area as the small cell. These large cells may belong to the same network as the small cell, or they may belong to another network operating in the same or an adjacent frequency band. The measure of these disturbances can be a bit error rate in digital communications systems. In analog systems, other signal quality measures can be used, such as total distortion of a modulated pilot tone.
The situation just described is illustrated in FIG. 2, in which a base station B 201 is associated with a cell 202, and a base station A 203 is associated with a cell 204. It will be observed that cell 204 is smaller than cell 202, and defines a geographical area that is located inside or near cell 202. Thus, because of the difference in the cells' relative sizes with respect to one another, cell 202 may be called a "macrocell" and cell 204 may be called a "microcell". When a mobile station B 205, associated with the "macrocell" 202 comes near base station A 203, the base station A 203 receiver may experience disturbances from mobile station B 205. This may be so even if the radio frequencies used by base station A 203 are widely separated from those used by base station B 201 and mobile station B 205.
If base station A 203 is equipped with a dynamically adjustable attenuator interposed between the antenna and receiver as described above, the attenuation can be increased until the disturbance experienced by the receiver is acceptably low. This is because the disturbances in the receive band are attenuated with at least the value of the attenuator. However, the desired signal from mobile station A 206, which is transmitting to base station A 203, is also attenuated with the value of the attenuator. This may cause the signal from mobile station A 206 to be unacceptably low for reception by base station A 203.